Improve socket:send/2,3 error handling for transient send backpressure

[harmon25]

This pull request improves the accuracy of error reporting in the socket send implementation, making it easier for callers to distinguish between transient backpressure and actual connection closure. The most important changes are:

Socket send error handling improvements:

• src/libAtomVM/otp_socket.c: Now returns {error, eagain} when a send operation encounters transient backpressure (such as lwIP ERR_MEM or BSD EAGAIN/EWOULDBLOCK), allowing callers to retry instead of misinterpreting it as a closed connection.
• src/libAtomVM/otp_socket.c: When the peer has closed the connection, returns {error, closed} instead of {ok, Data} to clarify the connection state.

Documentation update:

• CHANGELOG.md: Updated to reflect that socket:send/2,3 now correctly differentiates between transient backpressure and closed connections in its error reporting.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later